In the past, a robot provided with a movable or articulated arm has been used upon carrying a work, such as a silicon wafer used for manufacturing semiconductors, a glass substrate used for manufacturing a liquid crystal display panel, and the like. The robot of this type has a function that can be instructed in advance about a predetermined target position in order to accurately carry the work to the predetermined position.
In recent years, with an attempt to manufacture more large-sized semiconductor wafers and/or glass substrates, instruction to the robot has been more difficult and thus required higher skill of an operator. If the skill of the operator is not sufficient, serious mistakes may tend to occur upon instructing the robot. Therefore, there is a need for a technique for adequately instructing the robot about an accurate position, without relying on any highly specialized skill of the operator.
Patent Document 1 describes one method for obtaining an instruction point by detecting a position of a target by using a three-axis-scalar type robot. In this method, an end effector of the robot is moved toward the target attached to a cassette or the like, so as to be in contact with the target. At this time, changes in torque and speed are detected. Then, the detected changes are compared between the case in which the end effector is contacted with the target and the case in which the end effector is not contacted with the target, respectively. In this way, a contact point (or points) of the end effector with the target can be detected, thereby calculating the position of the target from the detected contact point, thus obtaining the instruction point.    Patent Document 1: U.S. Pat. No. 6,242,879
However, in the conventional instruction method described above, when the end effector is contacted with the target, the end effector and/or target may be deformed as well as unwanted particles may be generated. In order to avoid such problems, the robot should be moved at a very low speed. In such a case, however, accuracy of positional detection may tend to be significantly deteriorated due to various fluctuation factors and other factors that may be changed with time in a drive system of the robot. As used herein, the fluctuation factors include fluctuation due to the torque, friction and the like, while the factors that may be changed with time include hysteresis and the like.
In particular, in the case in which the target position in the vertical direction is instructed, some reaction force will be exerted on the end effector in a thickness direction (or vertical direction), from the target. Therefore, the end effector may tend to be deformed, thus making it difficult to detect the target position with high accuracy.
For instance, the end effector of the robot for carrying the semiconductor wafer is designed to access a wafer cassette produced based on the SEMI (Semiconductor Equipment and Materials International) standard. Therefore, the thickness of the end effector is limited within a range of from approximately 2 to 4 mm, and it would be quite difficult or impossible to apply the end effector having the thickness exceeding the above range to the wafer cassette based on the SEMI standard. In addition, based on the size of the semiconductor wafers manufactured in recent years, the length of the end effector should be within a range of from approximately 200 to 300 mm.
Namely, the end effector of the robot currently used for carrying the semiconductor should be sized to have an about 2 to 4 mm thickness and an about 200 to 300 mm length. Therefore, if assuming that the weight (e.g., 10 to 30 kg) of the robot supported in the vertical direction (or Z-axial direction) will directly correspond to the contact reaction force exerted on the robot from the target, it will be quite difficult to detect a signal without deforming the end effector. Namely, if such reaction force is exerted on the end effector upon the contact with the target, the end effector will be seriously damaged, thus deteriorating the accuracy of detection of the target.